The present invention relates generally to the television art and to the reception and displaying of transmitted signals.
In the United States, transmitted television signals are governed by the N.T.S.C. which prescribes the specific timing requirements for the signals. The horizontal synchronizing information sent in the form of sync pulses on the same carrier signal as the video information is governed by these standards. As shown in FIG. 1, the N.T.S.C. horizontal sync information shown as a pulse 12 occurs during a horizontal blanking period 14. The horizontal sync pulse 12 has a duration of approximately 4.7 microseconds. The pulse has an associated front porch 16 of duration approximately one microsecond and a back porch 18 of duration approximately five microseconds. Thus, the horizontal sync pulse 12 is not symmetrical with respect to the horizontal blanking interval 14.
The video information received from the transmitted signal is displayed on the television screen through a raster-like scanning fashion. That is, the scanning beam sweeps horizontally across the screen for the length of one line. It then "flies back" to the left hand side of the television screen. This "flyback" period has approximately the same duration as the horizontal blanking period, approximately eleven microseconds. A signal representative of the flyback signal also is shown in FIG. 1.
In operation, the horizontal sync pulse is phased-locked to the flyback pulse so that the fly back occurs once per horizontal line at approximately the same time as the horizontal blanking period. A phase locked loop will put the horizontal sync in the center of the flyback pulse. Because the horizontal sync pulse is not centered in the horizontal blanking period, the video will not be centered on the screen with respect to the flyback signal. Video information will still be being transmitted at a time when the flyback begins. Consequently, some of the video information which would normally be at the right hand edge of the screen will now appear off the screen during the flyback period.
The prior art has made attempts to rectify this problem. The most common method for attempting to produce a flyback signal which is symmetrical with respect to the horizontal blanking interval is to create a sawtooth waveform from the flyback signal. This can be done using the integration circuit of FIG. 2A which produces the associated signal of waveform (ii). The sawtooth waveform is then used to phase lock with the sychronizing pulse. Specifically, the phase detector will center the horizontal sync pulses about the rising edge at which the phase detector changes polarity, i.e. the point at which it switches from charging to discharging, as is shown in waveforms (iii) and (iv) of FIG. 2A. However, this does not solve the problem because the sync pulse will then lock onto the AC center of the sawtooth waveform which is identical to the center of the flyback, so the problem persists.
One method for overcoming this difficulty is to introduce a DC offset to the sawtooth signal. This can be done using the circuit of FIG. 2B which then produces the associated sawtooth, waveform (vi). In this circuit the AC center is moved back in the period of the waveform. Because of this, the synchronizing pulse which occurs during the early part of the horizontal blanking interval will lock onto an earlier portion of the flyback signal. Thus, the flyback will occur more nearly symmetrically with the horizontal blanking interval instead of the horizontal synchronizing pulse itself.
Generally, because of the offset introduced to overcome the misalignment of the flyback and the horizontal blanking interval, the phase detector's charge period increases and its discharge period decreases as is shown in waveform (vii) of FIG. 2B. As mentioned above, the horizontal sync pulse is centered by the phase lock loop about the edge at which the phase detector changes from charging to discharging. The first half of the sync pulse occurs during the charge period and the second half during the discharge period. Accordingly, the filter capacitor is maintained in equilibrium. And, because the offset has effectively moved the AC center to an earlier point in the sawtooth waveform's period, the sync now leads in phase with respect to the flyback which causes the flyback to be move nearly aligned with the horizontal blanking interval as is desired.
Although this prior art method theoretically solves the picture centering problem, it significantly degrades the noise immunity performance of the horizontal phase lock loop. Thus, it is unacceptable. Noise by its very nature occurs randomly and therefore is more likely to occur during the charge period of the phase detector because the charge period is now longer than the discharge period. Consequently, the phase detector filter capacitor will charge up and offset the phase lock loop causing it to drift away in response to the noise which has randomly turned it on.
Thus, it is the principal object of the present invention to provide an apparatus and method for horizontally centering a displayed television picture without introducing an unacceptable noise susceptibility into the system.
It is a further object of the present invention that such horizontal picture centering be accomplished with a minimum of discrete components. It is an associated object of the present invention that its features be accomplished with a minimum of power consumption and cost.
It is still a further object of the present invention that provision be made to protect against default conditions likely during manufacture or trouble-shooting situations.
It is still a further object of the present invention to allow for the use of a minimum number of integrated circuit pins in connection with the disclosed circuitry.